The present disclosure relates generally to the field of material science, and more particularly to controlling fragmentation characteristics of chemically strengthened glass.
Highly stressed glass has been known to fragment into small pieces since the Royal Society of London studied the phenomenon known as Prince Rupert's drops in the 17th and 18th centuries. There are several ways to create highly stressed glass. For example, tempered glass is a type of highly stressed glass that is made using thermal treatments. Tempering the glass puts the outer surfaces into compression and the inner surfaces into tension. The glass is placed onto a roller table, taking it through a furnace that heats it well above its transition temperature. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time. The outer layer wants to shrink as it is quickly cooled due to the glass's thermal expansion coefficient, but is unable to due to the higher temperature of the inner portion. This causes the outer layer to have large residual compressive stresses. As the interior of the glass slowly cools, it also wants to shrink due to the material's thermal expansion coefficient. Because the outer layer has solidified into shape, the inner region is unable to shrink. This causes the inner region to have large residual tensile stresses.
Another common way to create highly stressed glass, particularly for soda lime glass, is using chemical treatments, such as an ion exchange process. A commonly used ion exchange process for soda lime glass is a potassium and sodium (K/Na) ion exchange process. Unstressed glass is submerged in a bath containing a potassium salt, typically potassium nitrate (KNO3), at an elevated temperature. The sodium ions at the surface of the glass are replaced by potassium ions from the potassium nitrate. Because the potassium ions are roughly 30% larger than the sodium ions, the surface of the glass is put into a compressive state. The surface compression is balanced by residual internal tensile stresses. The exchange depth and the number of sodium ions replaced by potassium ions determine the compressive layer depth and the magnitudes of the compressive and tensile stresses.
Chemically strengthened glass' properties may be modified using ion irradiation. Ion irradiation (also referred to herein as ion beam irradiation) uses particle accelerators to emit charged particles (ions) towards a material to modify the material's properties. There are several types of ion irradiation techniques. One type, commonly used in semiconductor fabrication and the manufacture of silicon integrated circuits, is referred to as ion implantation.
In ion implantation, the ions may, depending on the energy at which they are emitted towards the material, penetrate the material to a depth before becoming stopped in the material (“implanted”), acting as an impurity. The ions may alter the elemental composition of the target. They also may cause many chemical and physical changes in the material by transferring their energy and momentum to the electrons and atomic nuclei of the target material. This may cause a structural change, in that the crystal structure of the target can be damaged or even destroyed by the energetic collision cascades. Because the ions have masses comparable to those of the target atoms, they knock the target atoms out of place more than electron beams do. If the ion energy is sufficiently high (usually tens of MeV) to overcome the coulomb barrier, there can even be a small amount of nuclear transmutation.